2.1. FERRITIN
Iron is known to be an essential element of the makeup of every living organism, but may also become toxic at physiological pH values by virtue of its tending to oxidize, hydrolyze and precipitate as insoluble ferric oxide polymers. The protein ferritin, found in all living cells, is the body's means for ensuring that iron toxicity does not occur. Ferritin functions by storing iron in the cells in a soluble and readily available form. The iron stored in cells may then be mobilized whenever needed by the body, for example, for erythropoiesis.
The name "ferritin" actually encompasses a number of individual isomeric forms which are characteristic of different tissue types. Each isoferritin has 24 subunits of two distinct types, namely light subunits (L) and heavy subunits (H). These subunits differ in molecular weight, the light subunit being about 18 kDa, and the heavy subunit about 19-21 kDa. The isoferritins extracted from different tissues or organs typically exhibit different isoelectric points, with the isoelectric focusing pattern of human tissues forming a continuous spectrum; those tissues associated with high iron storage have ferritins at the basic end of the spectrum (e.g. spleen and liver), while iron poor tissues, (e.g. heart and placenta) and malignant cells have acidic ferritins. (Drysdale, Ciba Found. Symp. 51:41, 1977). The difference in isoelectric point appears to be related to the different distribution of light and heavy subunits in each type. Specifically, heavy subunit-rich ferritins are relatively acidic, and light chain rich ferritins are relatively basic (Cosell, et al., in Ferritins and Isoferritins as Biochemical Markers, p. 49-65, 1984, Elsevier). Current studies indicate that the H and L subunits are encoded by a complex group of genes, therefore suggesting that there is an even greater heterogeneity of ferritin molecules than had previously been expected.